The present invention relates to a process for the purification of an inert gas containing impurities formed of organic compounds. The invention further relates to a process for the purification of an inert gas recycled from a polymerization reactor and particularly a solid-state polycondensation (SSP) reactor for aromatic polyester resins.
Polymer resins are molded into a variety of useful products. One such polymer resin is polyethylene terephthalate (PET) resin. It is well known that aromatic polyester resins, particularly PET, copolymers of terephthalic acid with lower proportions of isophthalic acid and polybutylene terephthalate are used in the production of beverage containers, films, fibers, packages and tire cord. U.S. Pat. No. 4,064,112 B1 discloses a solid-state polycondensation or polymerization (SSP) process for the production of PET resins.
While for fibers and films the intrinsic viscosity of the resin must generally be between 0.6 to 0.75 dl/g, higher values are necessary for molding materials such as containers and tire cord. Higher intrinsic viscosity such as greater than 0.75 dl/g can only with difficulty be obtained directly through polycondensation of molten PET, commonly called the melt phase process. The SSP process pushes polymerization to a higher degree thereby increasing the molecular weight of the polymer by the heating and removal of reaction products. The polymer with a higher molecular weight has greater mechanical strength and other properties useful for production of containers, fibers and films, for example.
An SSP process starts with polymer chips that are in an amorphous state. U.S. Pat. No. 4,064,112 B1 teaches crystallizing and heating the chips in a crystallizer vessel under agitation to a density of 1.403 to 1.415 g/cm3 and a temperature ranging between 230xc2x0 and 245xc2x0 C. (446xc2x0 and 473xc2x0 F.) before entering into the SSP reactor. Otherwise the tacky chips tend to stick together.
The SSP reactor may consist of a cylindrical reactive section containing a vertical mobile bed into which the polymer chips are introduced from above and a frusto-conical dispensing section at the base for dispensing the product chips. The polycondensation reactor typically operates at temperatures between 210xc2x0 and 220xc2x0 C. (410 and 428xc2x0 F.).
Various reactions occur during polycondensation of PET. The main reaction that increases the molecular weight of PET is the elimination of the ethylene glycol group:
PETxe2x80x94COOxe2x80x94CH2xe2x80x94CH2xe2x80x94OH+HOxe2x80x94CH2xe2x80x94CH2xe2x80x94OOCxe2x80x94PETxe2x86x92PETxe2x80x94COOxe2x80x94CH2xe2x80x94CH2xe2x80x94OOCxe2x80x94PET+HOxe2x80x94CH2xe2x80x94CH2xe2x80x94OH
An inert gas such as nitrogen is run through the polymerization reactor to strip the developing polymer of the impurities. The impurities present in the inert gas stream used in the production of polyethylene terephthalate in an SSP process generally include water and organics such as aldehydes and glycols, typically acetaldehyde and ethylene glycol and glycol oligomers. Also, volatile impurities include low molecular weight PET oligomers, such as the cyclic trimer of PET. Water is removed from the inert gaseous stream before it is recycled to the SSP because it can precipitate a reversal of the polymerization process. The organic impurities are removed to strengthen the polymer product and to assure that the impurities do not taint the compatibility of the end product with its use. Especially important is the prevention of organic impurities from leaching out of a resin container into the beverage contents. These impurities are stripped from polymer chips and accumulate in the inert gaseous stream. The organic impurities are present in the inert gaseous stream to be purified, in quantities, defined as methane equivalent, of about 2000 to 3000 ppm or more. U.S. Pat. No. 5,708,124 B1 discloses maintaining the ratio of inert gas mass flow rate to PET polymer solids mass flow rate to below 0.6 in an SSP reactor.
It is also well known that polyamide resins, and among them particularly PA6, PA6,6, PA11, PA12 and their copolymers, find wide application both in the fiber and flexible packaging sectors, and in the manufactured articles production by blow and extrusion technology. While the resin relative viscosity for fibers is low at about 2.4 to 3.0, higher relative viscosities of 3.2 to 5.0 are needed for articles produced by blow and extrusion technologies. The relative viscosity is increased to above 3.0 by means of an SSP process operating at temperatures of between 140xc2x0 and 230xc2x0 C. (284xc2x0 and 446xc2x0 F.), depending on the polyamide types used. U.S. Pat. No. 4,460,762 B1 describes an SSP process for a polyamide and different methods to accelerate this reaction.
An SSP process for polyamide resins is also described in the article xe2x80x9cNylon 6 Polymerization in the Solid State,xe2x80x9d R. J. Gaymans et al., Journal of Applied Polymer Science, Vol. 27, 2515-2526 (1982) which points out the use of nitrogen as a heating and flushing aid. The reaction is carried out at 145xc2x0 C. (293xc2x0 F.).
It is also known that the molecular weight of polycarbonate can be increased through an SSP process. Developing polyamides and polycarbonates also emit organic impurities that must be purged by an inert gas stream that must then be purified.
EP 0 222 714 B1 discloses a method for making polyethylene terephthalate and polyethylene isophthalate with very low generation of acetaldehyde to reduce the amount of purification required of the inert gas.
The conventional method used for the purification of an inert gaseous stream recycled from an SSP process includes an oxidation step for converting the organic impurities to CO2 and a drying step to eliminate the water formed in the polymerization process and the oxidation step. The oxidation step is carried out with oxygen or with gas containing oxygen, such as air, by using an oxygen concentration of no more than in slight excess of the stoichiometric quantity as regards the organic impurities. The oxidation step is controlled according to U.S. Pat. No. 5,612,011 B1 so that the inert gaseous stream at the outlet contains an oxygen concentration of not more than 250 ppm and preferably according to U.S. Pat. No. 5,547,652 B1 so that the inert gaseous stream at the outlet contains an oxygen concentration of not more than 10 ppm. These patents taught that a previously required deoxidation step of reducing the oxygen with hydrogen between the oxidation and drying steps was not required.
The oxidation reaction is conventionally carried out at a temperature between 250xc2x0 and 600xc2x0 C. (482xc2x0 and 1112xc2x0 F.) by circulating the inert gaseous stream over a catalyst bed formed of a support coated with platinum or platinum and palladium. The low oxygen content present in the inert gaseous stream exiting the oxidation section allows for recycling the same to the SSP process after the drying step. Moreover, higher oxygen concentrations in the recycled inert gaseous stream present the risk of oxidation reactions which degrade the polymer product, for example, by xe2x80x9cyellowingxe2x80x9d the product.
Japanese Publication 20885/71 discloses a method of reconstituting inert gas employed in the solid-state polycondensation or polymerization of linear polyesters comprising contacting the gas with one metal oxide at 150xc2x0 to 300xc2x0 C. (302xc2x0 to 572xc2x0 F.). The organic reaction products contained in the inert gas are oxidized to water and carbon dioxide. However, because the metal oxide loses its activity, it must be heated in the presence of air in a batch process. Accordingly, this publication does not pertain to a continuous catalytic gas purification process.
The last inert gas purifying step is a drying step carried out by circulating the gas over a silica gel, molecular sieves or other beds of drying materials. In this step, the water both stripped from polymer chips by the inert gas stream and generated in the oxidation step is eliminated. After this step, the inert gas is recycled to the SSP process. The small traces of oxygen, when present in the recycled inert gaseous stream, do not cause oxidation effects and/or polymer degradation. Even when the oxygen quantity in the oxidation reactor is stoichiometric or a little higher, it is possible to reduce the organic impurities to acceptable levels, such as less than 10 ppm defined as methane equivalent.
An article by E. V. Kuznetsova et al. entitled xe2x80x9cPurification of Industrial is Vapor-Gas Discharges and Wastewaters by Vapor-Phase Catalytic Oxidationxe2x80x9d discloses the use of platinum and other metal catalysts for the oxidation of organic substances in water vapor from a wastewater stream. The article indicated that as temperature went below 250xc2x0 C. (482xc2x0 F.), the degree of conversion of hydrocarbon substances is less than complete for the aluminum-copper oxide catalyst.
The platinum or platinum and palladium catalyst previously used in the purification of an inert gas from a polymerization process had to be run at 250xc2x0 to 600xc2x0 C. (482xc2x0 to 1112xc2x0 F.) to ensure adequate oxidation of the hydrocarbon impurities from the nitrogen gas stream when substantially stoichiometric quantities of oxygen are used. The higher temperature used in the reaction zone requires relatively costlier equipment and operation to preheat the impure inert gaseous stream fed into the oxidation zone. Moreover, greater equipment and operation costs were necessary to recover heat from the oxidation step.
Accordingly, an object of the invention is to provide a catalyst that will oxidize nearly all of the organic impurities from an inert polymerization reactor purge stream with substantially stoichiometric quantities of oxygen at lower temperatures.
It has been unexpectedly found that catalysts of 0.1 to 2.0 wt-% platinum in which the platinum is in a reduced state nearly completely oxidizes organic impurities from a polymerization reaction with a substantially stoichiometric quantity of oxygen at much lower temperatures than previously practiced, namely, below 250xc2x0 C. (482xc2x0 F.).
Accordingly, in one embodiment, the present invention relates to a process for the purification of a recycle inert gas stream leaving a polymerization reactor from organic impurities. The process comprises adding oxygen or a gas containing oxygen to the gas stream. The gas stream is contacted with a catalyst containing platinum, which has been subjected to reduction, at a reaction temperature of less than 300xc2x0 C. (572xc2x0 F.) in a reactor. The quantity of oxygen added is substantially stoichiometric with respect to the organic impurities such that a gas effluent from the reactor contains no greater than 250 ppm oxygen. The gas effluent leaving the oxidation reactor is dried to remove water from the gas effluent. The gas effluent is then recycled to the polymerization reactor.
In another embodiment, the present invention relates to a process for the purification of a recycle inert gas stream leaving a polymerization reactor from organic impurities. The process comprises adding oxygen or gas containing oxygen to the gas stream. The gas stream is contacted with a catalyst containing platinum in a substantially reduced state on a support at a reaction temperature of less than 300xc2x0 C. (572xc2x0 F.) in a reactor. The quantity of oxygen added is substantially stoichiometric with respect to the organic impurities such that a gas effluent from the reactor contains no greater than 100 ppm oxygen. The gas effluent leaving the oxidation reactor is dried to remove water from the gas effluent. The gas effluent is then recycled to the polymerization reactor.
In a further embodiment, the present invention relates to a process for the purification of a recycle inert gas stream leaving a polymerization reactor from organic impurities. The process comprises adding oxygen or gas containing oxygen to the gas stream. The gas stream is contacted with a catalyst containing platinum, which has been subjected to reduction, at a reaction temperature of less than 250xc2x0 C. (482xc2x0 F.) in a reactor. The quantity of oxygen added is substantially stoichiometric with respect to the organic impurities such that a gas effluent from the reactor contains no greater than 10 ppm oxygen. The gas effluent is then recycled to the polymerization reactor.
Additional objects, embodiments and details of this invention can be obtained from the following detailed description of the invention.